In recent years, global scale environmental destruction such as global warming, ozone layer depletion, acid rain, etc. has become an international problem. There is an urgent need for global environmental pollution countermeasures, and at present, from the viewpoint of environmental protection, various emissions regulations are enforced in each country. Among them, the problem of discharge of volatile organic compounds (VOC) such as organic solvents into the atmosphere is critical, and therefore, VOC reduction is being advanced under reinforcement of VOC emission regulations.
Heretofore, an organic solvent has been used for a coating material, but in an environment where VOC reduction is promoted, recently, a powder coating material has now become to be widely used. The powder coating material contains no organic solvent and thus requires no exhaust treatment or waste water treatment at the time of coating, and further can be recovered and reused, whereby the environmental load is extremely low.
In recent years, in various fields such as architecture, automobile, etc., there is a growing demand for a coating film with a hue of metallic tone. Usually for formation of a coating film with a hue of metallic tone, a coating material containing a metallic pigment such as aluminum powder, etc. (hereinafter also referred to as a metallic coating material) is used.
However, in a case where the metallic coating material is a powder coating material, as compared with the case of solvent-type or water-type, the metallic pigment tends to be hardly aligned parallel to the substrate, and luster of the coating film tends to be low, whereby there will be such a problem that no sufficient metallic hue tends to be obtained.
To alleviate such a problem, a bonding technique to attach a metallic pigment to the surface of powder particles (also referred to as a bonded method) is said to be effective (e.g. [0009] in Patent Document 1).
On the other hand, as the powder coating material, an acrylic resin-type powder coating material, a polyester resin-type powder coating material or an epoxy resin-type powder coating material has been mainly used. However, a cured film formed by such a powder coating material is inferior in weather resistance.
With respect to such a problem, for example, a powder coating material composition having the following (1) has been proposed.
(1) A layer separation type powder coating material composition which is a powder coating material obtainable by melt-kneading a mixture comprising from 10 to 50 parts by mass of a fluororesin, from 10 to 90 parts by mass of a polyester resin and from 10 to 50 parts by mass of a pigment, followed by cooling and grinding, and of such a type that when the powder coating material is melted and cured, the fluororesin and the polyester resin will be layer-separated without being compatibilized (Patent Document 2).
By using such a layer separation type powder coating material composition, it is possible to form, in one-coating operation, a coating film of a two-layer structure comprising a polyester resin layer on the substrate side and a fluororesin layer on the air side, and as the coating film has a fluororesin layer on the air side, it is said to be superior in weather resistance.